The invention relates to digital communication using packet switching and is particularly, but not exclusively, suitable for use in networks for transmission and diffusion of digital data representing sound signals, such as voice and high fidelity mono- and stereo-sounds.
While commercial development of packet switching networks is relatively recent, the general architecture and construction of such networks are well known and will not be described here. The techniques of data insertion, dynamic allocation, and data recovery used in transmission (i.e. communication between two well defined stations) and broadcast (i.e. between a sender and any number of receivers) are also well known and need not be repeated here in detail. If necessary, reference may be had to such documents as French Pat. No. 2,313,825; "Revue de Radiodiffusion Television", No. 40, Nov.-Dec. 1975 and No. 60, Nov.-Dec. 1979; and "Electronics", Dec. 20, 1979, pp. 89 seq.
The first step in the communication process comprises distributing the digital data into "packets", consisting of an opening part, called the prefix p, and an information useful part. The prefix provides indications defining the structure of the useful part. The useful part comprises a short length fraction of the message, which consists of a sequence of words each having x bits and representing the signal to be transferred. When the data are sound signals, the words typically each represent a quantized sample in digital form.
Reconstitution at the receiver is achieved by assembling a fixed predetermined number of words to form a unit which may be called a "digital frame", having a constant length l, having an opening portion consisting of a word of particular and identifiable construction, which will be called "frame lock word". That lock word makes it possible for the receiver to break up the received sequence into x-bit words correctly. The sequence includes the frame lock word at evenly distributed intervals.
The advantages of packet switching are known: each packet is an autonomous entity. Routing is transparent to the users and to the system used for transmission, storing and/or broadcasting the sequence of digital data. If the communication network has several alternative routes, packet switching leads to optimum use of the network capacity.
On the other hand, the data insertion techniques which are in present use (in which the only limitation is in respect of the maximum number of bits in a packet) result in a loss of continuity in the digital service if a packet is lost. Such a loss of a packet, related for instance to a faulty recovery of the data in the prefix, results in the loss of all data in the useful part of the packet. Since the length of the useful part is variable, the loss of a packet frequently results into a change of the periodical repetition of the digital sequence. The receiver detects an incorrect phase of the frame lock word with respect to its local clock. A resynchronisation procedure which implies searching for a structure corresponding to the frame lock word is necessary and the communication is broken during the search.
It is an object of the invention to improve upon the prior art packet data communication systems. It is a more particular object to reduce the loss of continuity in the service resulting from loss of a packet. For that purpose, the invention removes the need for a search of re-synchronization and timing by providing the data insertion device at said sender location with means for structuring the data as multibit words into frames having a fixed length l and each preceded with a frame lock word of identifiable constitution; and assembling said frame into short packets all having the same predetermined length L, with L=kl, where k is a predetermined integer programmed at said sender location and receiver location.
The arrangement of digital data in short packets having a fixed length L is rendered possible by implicitly programming the format of the packets in the multiplexer and demultiplexer provided for sending and receiving data, i.e. at both ends of the communication link (the word "implicit" meaning predetermined at both ends without any requirement for transmission and/or acknowledgment of commands prior to data transmission).
The above defined construction makes it possible to avoid any loss of information in addition to that contained in the useful part of the lost packet(s). The number of lost packets may be determined by inserting a continuity index word into the prefix of each packet or into the frame locking word at the sender location and then checking the continuity of the index words upon reception of the packets.
According to an ancillary object of the invention, when there is a certain amount of correlation between successive words, substitution words are generated in replacement of the lost words. The substitution words may be generated using as a basis the words before and after the lost words. Such an approach may particularly be used when the words represent successive samples of a sound signal. For that purpose, the process takes advantage of the fact the number of missing words may be computed, since each frame has a constant number of words and the lost packet has a constant and known number of frames (which however may be so assembled that a packet contains part only of the first and last frames).
The words, i.e. samples, which are generated and reinserted upon packet disassembly in the receiving facility or terminal are used to mitigate the effects of the loss of continuity in communication. If a single short packet, having k frames of each n words each representing a sample are lost, n.multidot.k samples are generated and inserted between the last sample N received before the communication is broken and the first sample N+nk+1 received after recovery.
According to another aspect of the invention, there is provided a process for digital data communication through a packet switching network, comprising: structuring the data into successive frames having a predetermined fixed length l; assembling the frames into packets all of them having the same fixed predetermined implicit format with a predetermined length L of the useful part of said packets with L=kl, k being a predetermined integer; inserting a continuity index word into the prefix of each packet or into the frame locking word of each frame; incrementing said index word by one each time a new packet or frame is assembled for transmission; transmitting said packets; and monitoring continuity of the index words upon reception of the packets prior to disassembly of the packets, said frames having a random location in said packets.
Particular embodiments of the invention will now be described by way of examples; since however the system may consist of individual electronic circuits of conventional nature, the block diagrams only will be fully defined.